The mutual coherence function that appears in the calculation of wave propagation in a turbulent atmosphere has never to our knowledge been derived in the Rytov approximation without appeal to the principle of energy conservation. We show why the Rytov approximation is not applicable beyond the first-order terms. A systematic perturbation based on the exponentiation of the Born series is carried out. Not only is energy conservation automatically satisfied in the new series, but also the results are consistent with other approaches.
© 1981 Optical Society of America
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